In graphic arts technology, a number of well-established printing processes utilize image carriers with three-dimensional (3D) representation of data, the most popular of them being flexographic printing, which uses flexible relief plates or sleeves. In a traditional flexographic prepress process with chemical etching there is no possibility of fine control of relief properties other than depth of relief.
Specifically, the shape of the cross-section profile directly influences the quality of reproduction of small features such as highlight elements and/or file linework details, process tolerance to changes in pressure applied by plate and/or sleeve to substrate and other vital characteristics.
Flexographic printing uses a flexible relief plate to print on a wide variety of substrates including paper, cardboard, plastic, and metal films. The Kodak Flexcel NX plate is one such relief plate. The process used to produce an image on the plate usually comprises the following steps:                1. Exposing the back of the plate to UV light.        2. Exposing an intermediate film to the desired image.        3. Laminating the film to the top of the plate.        4. Exposing the plate though the film using UV light.        5. Removing the film.        6. Using a solvent to wash away the unexposed plate material.        7. Applying additional exposure to harden the plate.        8. Drying the plate to remove as much of the solvent as possible.        
The back exposure is used to establish the floor of the plate. The intensity of the exposure decreases as the illumination penetrates the plate because of absorption in the plate material. Once the intensity drops below a threshold value, there is insufficient cross linking in the polymer comprising the plate and the remaining under-exposed polymer can be washed away. This is usually the top 0.5 mm of the plate. To form the relief, the front of the plate is exposed, through an image layer, with enough intensity that sufficient cross linking occurs all the way down to the plate floor.
For every opening in the image layer, a cone of UV light with an angle of about 40 degrees from the normal propagates through the plate forming cone shaped relief dots. A cross-section of a plate 500 is shown in FIG. 5. The following features are depicted in the cross-section 500: a solid area 504; an isolated dot 508; and an array of closely spaced dots created by a halftone screen 512. The height of the plate relief is shown by numeral 516 and plate floor by numeral 520.
Isolated dots, such as isolated dot 508, can be problematic. There may be insufficient exposure to solidly and anchor the dot to the plate floor 520. Even if the dot forms properly, excess printing pressure could cause the dot to deform during printing. The dots in the middle of the halftone array 512 fair better since they are supported to either side by nearby dots. However, dots at the edge of the array 512 could suffer from some of the same problems as the isolated dot 508. Dot deformation can cause a large objectionable blot to form on the printing substrate. This is called a scum dot in the industry. Ensuring good dot formation and eliminating the possibility of scum dot formation is the object of this invention.
Large dots can support themselves even in isolation. For the Flexcel NX plate, a minimum dot size of 4×4 pixels is usually sufficient to ensure proper dot formation in all cases. However, a large minimum dot in a halftone makes it difficult to print light grey tones. Bump curves or screening strategies are used to try to mitigate this problem with mixed success.
The typical plate relief is 20 mils (0.5 mm). Reducing the relief, improves the dots ability to stand on its own. The disadvantage is that over a long print run, dirt can collect in the wide areas of the floor and if sufficient dirt accumulates then this dirt will transfer to the substrate.
A method that has been successfully used in laser ablation mask plates (LAMS) is to deliberately expose dots 608 that are too small to properly form in the areas surrounding dots 604 that need additional support as is shown on a printing plate profile 600 in FIG. 6. There is some risk the dots may print despite the lower relief—ink may accumulate on the recessed dots over several print cycles and then transfer to the next substrate all at once. Debris accumulation may also be a problem.
Another method suggests small dots which are interspersed with large dots. Halftone screen 700 as is shown in FIG. 7. As halftone dots 704 become sparse, rather than remove a dot completely a halftone dot is replaced with small printing dot 708. This is not ideal but often the tonal value of the resulting halftone screen is less than the sparse array because the remaining dots have additional support.